1. Field of the Invention
The present invention relates to an internal resonator type of laser device which generates a second harmonic.
1. Description of the Related Art
In a laser device in which a nonlinear optical crystal is inserted into a laser resonator to generate a second harmonic with a high efficiency, wherein the laser device has a plurality of longitudinal modes, there will be generated a sum-frequency between the longitudinal modes as well as a second harmonic on each of the plurality of longitudinal modes of laser beams. It happens that a generation of the sum-frequency causes strong mode competition noises, called a "green problem", to be generated. It has been desired from various application sides that the "green problem" is solved, and hitherto, there has been proposed several solutions to this problem.
FIG. 9 is an illustration of low-noise operation of a laser according to the earlier technology. Such a laser is equivalent to the internal resonator type of solid state laser device which generates a second harmonic of low noises, as disclosed in the document: Michio Oka and Shigeo Kubota, Optics Letters, Vol. 13, No. 10, P.805 (October 1988).
FIG. 9 shows the configuration of the laser cavity comprising: an Nd:YAG crystal 31 serving as the laser medium; a KTP crystal 32, that is, a Type II phase-matched nonlinear crystal, which generates the second harmonic of the laser beam having the wavelength 1064 nm emitted from the Nd:YAG crystal 31; a quarter-wave plate (QWP) 33 inserted at the laser beam having the wavelength 1064 nm emitted from the Nd:YAG crystal 31, the fast axis of the QWP 33 being arranged with angle 45.degree. with respect to the extraordinary axis of the KTP crystal 32; and an output mirror 34 for outputting the second harmonic generated in the KTP crystal 32, the output mirror 34 constituting a laser resonator.
According to the laser mentioned above, the fast axis of the QWP 33 is arranged with angle 45.degree. with respect to the extraordinary axis of the KTP crystal 32. Thus, the eigenpolarization modes of laser beams in the resonator are controlled to cancel a nonlinear polarization which causes a sum-frequency to be generated in the KTP crystal 32. As a result, there is generated no sum-frequency and thus no noises due to the green problem.
FIG. 10 is an illustration of low-noise operation of a laser according to the earlier technology, which is different in system from that shown in FIG. 9. Such a laser is equivalent to the solid state laser device as disclosed in the document: Hideo Nagi, et al, IEEE, Journal of Quantum Electronics, Vol. 28, No. 4, P. 1164 (April 1992).
FIG. 10 shows a laser system comprising: an Nd:YAG crystal 41; a laser diode 36 for emitting excitation light 37; a lens 38 for focusing the excitation light 37 to be introduced into the Nd:YAG crystal 41; a KTP crystal 42, that is, a Type II phase-matched nonlinear crystal, which generates the second harmonic of the laser beam emitted from the Nd:YAG crystal 41; and a Brewster plate 43 arranged in such a manner that the angle between the field direction of polarization, in which the highest transmission is obtained at the plate and that of the extraordinary ray in the KTP crystal 42 is 45.degree..
According to this laser system, the well known birefringent filter is formed in view of the fact that the KTP crystal 42 has a birefringence property and a transmission of the Brewster plate 43 depends on the direction of polarization, so that the different resonance loss is given on each of the longitudinal modes of the laser beams. Consequently, only a certain one longitudinal mode, which is involved in the lowest loss, is selectively effective for an oscillation. Hence, a sum-frequency is not generated. Thus, it is possible to suppress the noise generation. As a scheme in which a laser is oscillated in a single longitudinal mode, there is also well known one using a ring type resonator.
FIG. 11 is an illustration of low-noise operation of a laser according to the earlier technology, which is different in system from that shown in FIGS. 9 and 10. Such a laser is equivalent to the laser device as disclosed in the document: W. L. Nighan et al, Technical Digest of Advanced Solid State Lasers, Post Deadline PD4 (1996).
FIG. 11 shows a laser system comprising: an Nd:YVO.sub.4 crystal 45 which is a laser medium having a wide oscillation wavelength width; a laser resonator constituted of mirrors 51-57; and two excitation optical systems each comprising an optical fiber end 46, a collimating lens 48 and a condenser lens 49. Excitation light emitted from each of the excitation optical systems is fed to the Nd:YVO.sub.4 crystal 45 passing through the mirror 52 or 53.
The laser resonator includes: an LBO crystal 40 for generating a second harmonic of the laser beam generated in the Nd:YVO.sub.4 crystal 45, the LBO crystal 40 being a non-linear optical crystal; and a lens 41 for controlling a laser mode of the LBO crystal 40. According to this laser system, the Nd:YVO.sub.4 crystal 45, which is a laser medium having a wide oscillation wavelength width, is used and the laser resonator is extent up to about 1 m by means of doubling the light path using the mirrors 54, 55 and 56, so that a number of longitudinal modes of the laser oscillation light is increased to about 100 axial modes. Thus, a fluctuation of the longitudinal mode output, which is caused by the green problem of the second harmonic generated in the LBO crystal 40, is decreased by mutual cancellation of fluctuations of outputs of a large number of longitudinal modes. Consequently, it is possible to apparently reduce the noises on the overall output of the laser system.
However, according to the scheme shown in FIG. 9 in which quarter-wave plate is inserted, it is needed that two orthogonal polarization modes are oscillated in the associated single longitudinal axial modes, respectively. And a tolerance as to a reflecting power of the coating of the respective crystals and an alignment angle of the mirrors is narrow. Further, in order to maintain the noise-less state for a long time, there is a need to perform a temperature control for the respective parts and the resonator in its entirety with great accuracy.
Also in the scheme shown in FIG. 10 in which the Brewster plate is disposed, in order to stably maintain a single longitudinal mode, there is a need to control a temperature of the KTP crystal and a resonator length with great accuracy. Further, in the event that it is desired to obtain a high output power over 1 w, a high gain of laser medium is required. This makes it easy to bring about a plurality of longitudinal modes on the laser beam. The plurality of longitudinal modes causes noises due to the green problem to be generated. Thus, it is feared that an operational stability of the laser is damaged.
According to the laser system shown in FIG. 11, the resonator length is elongated and a number of longitudinal modes is established above 100 axial modes. This causes an oscillating wavelength band to expand. Thus, it is feared that the wavelength conversion efficiency is lowered since there is a limit in wavelength tolerance of the phase matching. Further, a distribution of the longitudinal modes is easily influenced by reflection of the optical elements or the like inserted into the resonator, and there is the possibility that the longitudinal modes each associated with mutually different frequencies are oscillated. In this case, it is feared that noises due to the green problem are generated. Furthermore, it is difficult to provide a miniaturization of the device because there is a need to provide a resonator length not less than 1 m. And in addition, the resonator is complicated in structure. Thus, it is feared that the mechanical stability is lowered. It is noted that it is very important for assembling parts into a laser device that an adjustment is easy and a degree of freedom in design is large.